SPECIATION - StudyTime NZfew familiar faces like natural selection, genetic drift, Bottleneck...

NCEA | Walkthrough GuideLevel 3BIOLOGY

SPECIATION

Introduction 3

Defining Evolution 4Processes of Evolution 5

Gene Pool and Allele Frequency 5Genetic Drift 7Founder Effect 9Bottleneck Effect 10Migration 11Types of Selection 13Polyploidy 15

Speciation 17

Defining a Species 18Allopatric Speciation 19Sympatric Speciation 20Reproductive Isolating Mechanisms (RIMs) 23Pre-zygotic RIMs 24Post-zygotic RIMs 27

Patterns of Evolution 29

Divergent Evolution 29Convergent Evolution 32Co-Evolution 33Gradualism and Punctuated Equilibrium 34Adaptive Radiation 35

Key Terms 39

Level 3 Biology | Speciation Cram Guide

Level 3 Biology - Speciation | © Inspiration Education Limited 2017. All rights reserved.3

INTRODUCTIONThis standard is all about evolution.

No doubt you’ve heard the word ‘evolution’ being thrown about all over the place (Charles Darwin and humans evolving from apes and all that) but what is it actually all about, and how does it occur?

This external standard builds on all concepts you learnt last year in Level 2 Biology: Genetic Variation and Change – hopefully you remember one or two things.

Join us as we try to figure out how Earth managed to get 9 million different species living on it, all originating from one, boring, unaware single-celled organism 4 or 5 billion years ago.

What will you learn in this walkthrough guide?

We’ll kick things off with all the different ways we can get changes in allele frequencies – and don’t worry, we’ll define “allele frequency” as well. This will include revisiting a few familiar faces like natural selection, genetic drift, Bottleneck effect, Founder effect and migration. To keep things interesting, we’ll introduce you to a couple of new forms of selection as well!

Next up will be speciation. This covers how one species evolves into another - or how we can end up with a whole bunch of different species from a common ancestor. The main two things that will be covered will be sympatric and allopatric speciation.

Finally, we’ll consider the “patterns of evolution”. This includes a few different types of evolution: divergent evolution, convergent evolution, co-evolution and adaptive radiation.

A word on exam strategy.

As with lots of biology, Speciation is a pretty jargon-intensive topic, with a whole bunch of definitions. The key is to push through that phase of “OMG, are they even speaking English” and try and break down each word one-by-one.

Here at StudyTime, we’re pretty much GCs (good citizens), so to help you out, we’ve made this guide in plain English as much as we can. We’ve also included a glossary for some of the key terms that you’ll need to master for your exam.

4 Level 3 Biology - Speciation | © Inspiration Education Limited 2017. All rights reserved.

Level 3 Biology | Speciation

However, the language we use isn’t always something you can directly write in yourexam. When this is the case, we offer a more scientific definition or explanation (in ahandy blue box) underneath. These boxes are trickier to understand on your first read through, but contain language you are allowed to write in your exam. Look out for them to make sure you stay on target.

DEFINING EVOLUTIONYou’ve probably heard it dropped in conversation - but what does ‘evolution’ actually mean?

Evolution can simply be defined as a change in the gene pool of a population over time.

Recapping what you learnt last year, this means a change in the frequency of different alleles.

Gene Pool

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These alleles arise from mutation. Just remember that mutations happen randomly at approximately the same rate. If the mutation causes an allele change, some of the corresponding phenotypes are selected for or against.

Over time, this becomes evident in the heritable characteristics, or traits, in the population.

Whether you’re talking about individual alleles - or characteristics across a giant population, evolution is the idea that all life originated from a common ancestor.

It is the idea that, over time, differences in evolution became responsible for all the biodiversity among species, individual organisms and even biological molecules.

Our mate, Charles Darwin, described evolution by natural selection

Natural selection is another name for ‘survival of the fittest’.



Thinking about this, natural selection is a process where organisms that are better adapted to their environment due to their genetics are more likely to survive, reproduce, and pass on their traits to their offspring. Resulting in more organisms with the same genetic traits as them in future.

Population Change Over Time

Generation 1 Generation 2 Generation 3

Evolution is the core of this external standard, and so we’ll look at how it occurs (processes of evolution) as well as the types of evolution that can occur (patterns of evolution).

STOP AND CHECK:

See if you can come up with a good definition for evolution in your own words.

PROCESSES OF EVOLUTIONWhen we are discussing genetic change, we are referring to changes to the frequency of alleles in a gene pool.

Natural selection, mutations (both conveniently covered in Level 2 Biology), migration, genetic drift, Founder effect and Bottleneck effect can all cause changes to allele frequencies in populations.

You will need to be able to discuss all of these processes and be able to compare and contrast between them.

By the end of all of this you should know a handful of things, including:

The importance of gene pools and allele frequency, and how we will use them to explain evolution. How genetic drift, bottleneck effect, founder effect and migration can change the allele frequencies in a population’s gene pool. The difference between each of the 3 types of natural selection: stabilising, disruptive and directional selection. An exciting new topic: instant speciation.



Gene Pool and Allele Frequency

From level one, we know that, within a population, different individuals have different combinations of alleles - depending on what they inherited from their parents. Back in the day, we told you to call this combination of alleles the genotype.

Imagine that we could somehow take a list of every single gene in every person in New Zealand and count up how many of each type of allele is present for each gene in the population.

The gene pool is the total set of alleles that are present within a population. In other words, The alleles present in the gene pool are all of the alleles that could possibly be passed onto the next generation.

Sample Population Frequency of Alleles

allele forbrown hair

allele forgrey hair

heterozygousgrey hair

homozygousbrown hair

homozygousblack hair

Not only can we count up all the alleles that make up the gene pool…

…but we can think about which alleles are more common than others

The number of one allele relative to the total number of all alleles for that gene – or the proportion of one allele – is the allele frequency.

It is simply calculated by taking the total number of one allele for a particular gene and dividing this by the total number of alleles in the gene pool for that same gene.

For example, if you had forty mice in a population, and ten of them had one allele for brown coat, and fifteen of them had two brown coat alleles, the allele frequency for brown coat would be:

10 + 15 × 240 x 2 = 0.5

The idea of allele frequencies is super important because it forms the basis of evolution. As the frequencies of alleles change - so too do the heritable characteristics (or traits) in a population.



Changing the allele frequencies is necessary to create evolutionary change in a population

Linking this to natural selection, we know that if the allele frequency for one allele increases over time, it is likely it is being selected for. This means it is either increasing the organism’s chances of survival and/or chances of reproduction.

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STOP AND CHECK:

Turn your book over and see if you can remember:

The definition of a gene pool. How to calculate an allele frequency.

Try to explain it in your own words.

Genetic Drift

Just like natural selection, we know from level two that genetic drift results in an increase in frequency of some alleles - and a corresponding decrease in others.

In natural selection, these changes in frequency happen for a reason:

That is, the advantageous alleles increase in frequency and the less or disadvantageous alleles decrease in frequency.

People tend to understand natural selection, because it makes a lot of sense.



Genetic drift can be harder to come to terms with

That’s because it is the change in allele frequencies due to chance.

So, what kind of chance events cause genetic drift?

In level two, we talked about how, due to meiosis, every gamete ends up with a different combination of alleles. When a sperm and an egg meet, it is completely random which combinations of alleles happen to be in that particular sperm and that particular egg.

That means that it is also random which of the parents’ alleles get passed onto the offspring and which don’t.

Millions of these chance events over time can add up to the allele frequencies of the gene pool changing, for no selective reason at all. The effects of genetic drift are much greater in smaller populations.

Larger Population:

Smaller Population:



STOP AND CHECK:


The process of genetic drift and how it changes the make-up of the gene pool. What determines the effect that genetic drift has on the population.


Founder Effect

Imagine in a large population there is a small group of individuals who decide to leave and live somewhere else.

In nature, this happens all the time. Small groups of a population might leave due to competition, due to getting lost or left behind, or because they are separated from the larger population by a freak accident.

Mother population New population

Founder Effect

Either way, they leave, set up their own camp, and make up their own population.

This is known as the Founder Effect

The founder effect involves a small group of individuals moving away from the main population, and therefore establishing their own gene pool.

One of two things will happen when the small group breaks off:

1. The small population has a similar gene pool to the original larger population.2. The small population has a different gene pool to the original larger population.

Chances are, the second option is most likely to occur

That’s because the small population is just a small sample of the original population. It’s likely that the ratio of alleles will be different. As well as this, certain alleles that



were in the original population may not be present in this new group.

Alternatively, every individual with a particular allele from the main population may have moved to the ‘founding’ group - resulting in the original gene pool losing an allele.

Not only is it likely that the small population has a different gene pool, it is likely that it has reduced genetic diversity compared to the original population.

On top of this, genetic drift (which we just spoke about) has a larger effect on smaller populations. This means that the new populations are now more likely to face further shifts in allele frequency due to their decreased size!

Large pool Small pool

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Geneticdrift occurs faster

in smallpopulations

STOP AND CHECK:


What happens during the Founder Effect. Why the gene pool of the founding population (the small group) is likely to be different to the larger population.


Bottleneck Effect

Picture a bottle:

A bottle starts off large at the bottom and then suddenly there is a narrowing at the top.



In the Bottleneck Effect, we start with a large population, which becomes much smaller. This could be due to a sudden change in climate, disease, loss of habitat, or maybe even HUMAN INTERACTION *shocked gasp*.

Let’s think about the effect on genetic diversity

When a population greatly reduces in size, the small remaining population is often left with reduced genetic diversity. At the very least, there is a change in the make-up of the gene pool, as the small populations makes up just a small sample of the original population. This is the bottleneck effect.

Often alleles may be lost from the population as well.

Original population Bottlenecking event Surviving population

At this point, the same effects occur as with the Founder Effect: the population is more prone to genetic drift. Inbreeding is also likely, which decreases genetic diversity of the offspring.

STOP AND CHECK:


What happens to the size of the population during the Bottleneck Effect – what might cause this change?

The effects the Bottleneck Effect has on the genetic diversity. Why genetic drift has a greater effect on the population after Bottleneck Effect occurs.




Migration

Migration is the movement of individuals from one population to another.

The word immigration is probably very familiar to you. You hear it used to describe people moving into one country from another country.

If you are the person moving out of a country to another country, then you are emigrating.

emigration

Population I Population II

immigration

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Both immigration and emigration are just types of migration: moving from one population to another.

Can you work out how migration changes the allele frequencies of populations?

When individuals move into a population, their alleles are added to the population’s gene pool.

The allele frequencies in the other population might be different, so it might be that the new immigrants all have a particular allele which is rare in the population they have arrived in. So, when their alleles are added to the gene pool of their new population (and when these individuals begin to reproduce), the allele frequencies change.

Gene flow

Population 1 Population 2



Similarly, when individuals leave a population, they take their alleles with them, out of the gene pool. If most of the individuals that leave have a particular allele, then their migration results in a change in the allele frequencies of the population left behind.

STOP AND CHECK:


What migration is: what’s the difference between immigration and emigration? The effect that migration has on the gene pool of a population.


Types of Selection

In Level 2 Biology we had a look at natural selection – which was also described earlier on in this Cram Guide. Since this is Level 3 Biology it’s only right that we make the idea just an extra bit more complicated with the idea of modes of selection.

Before we jump into modes of selection, it’s important to yarn about phenotypic range

Take height, for example. Height is a complex phenotype where individuals can be short, tall or anywhere in between. In a population, there will be a distribution showing the number (or proportion) of individuals with each height.

For most complex phenotypes, there will be a very small number of individuals with the extreme phenotypes (such as being gigantically tall or microscopically small) and larger numbers of individuals with the average, or middle, phenotype.

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Time for these different types of selection:

We can describe different types of selection based on how the selection affects the



phenotypic distribution in a population.

1. Stabilising Selection

What do you think of when you hear the term “stabilising”. It kind of seems like things are being evened out making things more ‘normal’.

In stabilising selection both extremes of the phenotypic range are selected against, while the middle phenotype – or the average – is selected for.

Over generations, the number of individuals with the extreme phenotypes decrease, while the number of individuals with the middle phenotype increases. It’s like things are evening out.

Selection against both extremes

Original population

Population after selection

2. Disruptive Selection

Disruptive selection is the opposite of stabilising selection.

Rather than trying to balance things out and keep the phenotype normal, disruptive selection decides to make things interesting and focus on the extremes instead. In disruptive selection, both extremes of the phenotypic range are selected for, while the middle phenotype – or the average – is selected against.

Over generations, the number of individuals with the middle phenotype decreases.

Selection against the mean

Original population


You can imagine that if disruptive selection is really dramatic the population might be split into two:



Here, one population with one extreme end of the phenotypic range, and the other population with the other extreme end of the phenotypic range.

Therefore, disruptive selection can lead to speciation as it splits the population into two groups with distinct phenotypes and phenotypic range. It leads to speciation as natural selection is acting on different sets of alleles.

3. Directional Selection

As the name suggests the phenotypic range is going to move in one direction or the other.

In directional selection, a single phenotype is selected for.

Over generations, this increases the number of individuals with that phenotype, shifting the whole phenotype range in one direction.

Selection against an extreme

Original population


STOP AND CHECK:


How to describe a phenotypic range. The 3 modes of selection and the differences between them.


Polyploidy

The ploidy of a cell refers to the number of chromosomes it contains. Changes to the number of sets of chromosomes can lead to what is referred to as “instant speciation”, which has been important to the evolution of many plants.

Polyploidy refers to variation in the number of chromosome sets.



In humans, we have two copies of each chromosome, thus two sets of chromosomes. As a result, we are referred to as diploids.

In plants, it is common for there to be 3+ sets of chromosomes. These plants are referred to as polyploids.

Haploid (n) Diploid (2n)

Triploid (3n) Tetraploid (4n)

How does polyploidy occur?

Polyploidy is the result of non-disjunction during cell division.

In non-disjunction chromosomes fail to separate (disjoin) during division. Instead of moving to opposite poles of the cell, a pair of chromosomes moves to the same pole. As a result, one daughter cell will have more chromosomes than normal and one daughter cell will have less chromosomes than normal.

If a polyploidy gamete is fertilised with either a normal or polyploidy gamete, the offspring may be polyploid.

There are two types of polyploidy:

1. Autopolyploidy2. Allopolyploidy

Autopolyploidy is where the genome is multiplied within a single species, while allopolyploidy results from the hybridisation between species.

How does polyploidy result in “instant speciation”?

When an individual (individual plant) from a particular species has a duplicated,



or multiplied, set of chromosomes, they can no longer breed and produce viable offspring with members of their original species, due to hybrid sterility.

Because there are many plants which can self-fertilise, the polyploid is able to be maintained in the environment.

AllopolyploidDiploid

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Tetraploid

Diploid

AutopolyploidDiploid

Triploid Tetraploid

STOP AND CHECK:


The definition of polyploidy. The difference between autopolyploidy and allopolyploidy. How polyploidy arises. What is meant by the term “instant speciation”?


Autopolyploidy



? Quick Questions

The four-wing saltbush plant is a shrub that has undergone polyploidy. It has a haploid number of 9 chromosomes. But there are four-wing saltbushes with different numbers of chromosomes, and these different types of saltbush live in a slightly different habitat depending on water availability.

Describe polyploidy and describe why the four-wing saltbush polyploids are fertile. Explain how polyploid formation might have occurred in the four-wing saltbush.

SPECIATIONSpeciation describes the formation of new species as populations become reproductively isolated, preventing gene flow. Speciation can occur gradually as small changes accumulate over time, or it can occur instantly through changes in the chromosome set numbers (polyploidy).

There’s a few things that need to be covered in this section:

Can’t talk about speciation without knowing what a species is – we better define it! Speciation can be split into two types based on how these populations are isolated. We’ll compare these two types of speciation. Speciation requires some isolation of gene flow. You’ll need to be able to define these reproductive isolating mechanisms and give examples of them.

Defining a Species

You’d think that with all this information about evolution and speciation that actually defining what a species is would be easy. On paper, it is easy to define: “a group of organisms containing individuals which can produce fertile offspring”. These fertile offspring are individuals that can also reproduce.

Unfortunately, trying to classify a species in the real world is quite niggly

First of all, there is the problem of hybridisation where organisms produced by two different species can sometimes be fertile.



Lion

+Tiger

Another issue is ring species, where there is a series of neighbouring populations which can interbreed, but the populations at the ends of the series are too distantly related to interbreed. In all these cases, the cut-off for defining a species becomes very blurry.

Successfullyinterbreed

Can notinterbreed



Successfully interbreed

How does this link to speciation?

Of course, species may change due to evolution - which is what we’re about to look at in the next few sections. This evolutionary process where populations evolve and become different, distinct species, is called speciation.

STOP AND CHECK:


The definition of a species. The reasons why it is difficult to define a species in real life. What speciation is.




Allopatric Speciation

There are two main types of speciation that differ in how the populations are isolated from one another:

1. Allopatric speciation2. Sympatric speciation

First up is allopatric speciation

Remember, in all cases of speciation we have at least two populations belonging to the same species. These separate populations then evolve into distinct species over time.

With allopatric speciation, the separate populations become geographically isolated

What this means is that there is something that is keeping them in different geographic locations.

For example, a river might appear and block the populations from being able to mix and mingle. Another example could be mountains popping up which separate the two populations.



Because the populations are in separate geographical locations they will have different selection pressures to face

Using an extreme example, a mountain formation might disrupt the climate so that one population faces hot weather while the other faces cold weather. Over time the heritable traits in each population will change until the populations have become so different that they are now distinct species.

Another example could be that a river formation blocks off one population’s normal food source, forcing them to have a different diet.

These different selection pressures will select for, and select against, different traits.

The geographic isolation is very important

In order for these two populations to become separate species there can be no interbreeding between them throughout this time; the gene pools must stay isolated.This is why there needs to be geographic barriers, such as rivers or mountains, that stop them from being able to mix and mingle.

Therefore, the initial step in allopatric speciation is barrier formation.

STOP AND CHECK:


The requirements for allopatric speciation. Why geographical isolation is important in allopatric speciation.




Sympatric Speciation

Sympatric speciation is the complete opposite of allopatric speciation…

…so if you remember just one you’ll remember them both.

While allopatric speciation requires geographical isolation, sympatric speciation involves populations becoming distinct species within the same geographic location.

This means that they live in the same area; how will this work?

The easiest example of sympatric speciation is with instant speciation of plants due to polyploidy. The polyploid offspring, which represent a new species, are able to occupy the same geographic location as their parents, and so we have sympatric speciation.

Other examples include certain populations of insects which end up feeding on different food sources due to competition. This is enough of a difference in selection pressures to become different enough to become distinct species.

Alternatively, there may be a preference by some organisms to breed with particular types of individuals of the same species, leading to two separate breeding populations that could potentially form distinct species within the same location.



Whatever the example, there are 3 things that are required for sympatric speciation:

1. The populations that evolve into separate species occupy the same geographical location.

2. The populations face different selection pressures that select for different traits, allowing the populations to become different enough from one another. This can include differences in diet, sexual preference, anything like that.

3. There needs to be some kind of barrier that stops the different populations from interbreeding.

The 3rd point is important

For any speciation to occur we can’t have interbreeding occurring.

If there is interbreeding between two populations they won’t become genetically different enough to form separate species. Even though they’re living in the same location, sympatric species need some kind of barrier that stops them from breeding.

OriginalSpecies

OriginalSpecies

OriginalSpecies

If the populations have different food sources, this might stop them from mixing and mingling, or if there are differences in sexual preference this will stop them from breeding with members of the other population.

What causes different members of the species to change their diet or to have a different sexual preference?

Ultimately it comes down to some kind of genetic mutation or general genetic difference.

Even if the different diet is due to competition, there will be genetic similarities between the members of the population eating one food source which is different to the population forced to eat a different food source. Same with differences in sexual preference.

Genetic mutation is the initial step of sympatric speciation.



STOP AND CHECK:


The difference between allopatric and sympatric speciation. The 3 requirements for sympatric speciation to occur. Why polyploidy can cause sympatric speciation. The initial step of sympatric speciation.


Reproductive Isolating Mechanisms (RIMs)

In both types of speciation, we mentioned that there needs to be something that keeps the different populations isolated long enough for speciation to occur.

These things or barriers that keep the populations apart are called reproductive isolating mechanisms (RIMs). They are defined as any factors which prevent individuals from different populations breeding.

Ultimately, these RIMs prevent gene flow between populations.

There are two types of RIMs:

1. Pre-zygotic RIMs2. Post-zygotic RIMs

PrezygoticBarrier

Sperm

egg hybridPostzygoticBarrier

We’ll explore each type in the next two sections.



STOP AND CHECK:


The definition of reproductive isolating mechanisms (RIMs). Why RIMs are important to speciation. The two types of RIMs.


Pre-zygotic RIMs

Pre-zygotic means before the zygote is formed

Therefore, pre-zygotic RIMs act before any babies can be made, or more precisely before fertilisation of the egg occurs.

As with all RIMs they prevent gene flow.

There are a number of different pre-zygotic RIMs

The common feature of all of them is that they stop two individuals coming together and breeding.

The first type of pre-zygotic RIM is present in allopatric speciation

It is geographic isolation, where physical barriers, such as rivers and mountain ranges, can prevent individuals from different populations from coming together and mating.

The other pre-zygotic RIMs may be present during sympatric speciation when they are in the same geographic location

But we can break these down into pre-zygotic RIMs that depend on the physical differences between the individuals and those that do not.



There are a few structural features that act as RIMs

First of all, the structures required for mating can become too different over time, which physically prevents fertilisation between individuals.

For example, even though they all belong to the same species, some breeds of dog can’t breed with others – if you try to cross a Great Dane with a Chihuahua you’re going to be quite disappointed.

This kind of pre-zygotic RIM is called structural, or morphological, isolation.

The other pre-zygotic RIM that depends on the physical features is gametic isolation

This is where the eggs from one population are incompatible with the sperm of the other, preventing fertilisation from occurring.

Maybe the sperm can’t survive in the female’s reproductive system or maybe it’s not strong enough to push through the surface of the egg.

These aren’t the only RIMs that exist

The last set of pre-zygotic RIMs simply make it unlikely that individuals will come across one another to mate, even though they live in the same area.

Examples include, ecological isolation, temporal isolation and behavioural isolation.

Ecological isolation:

Ecological isolation is due to differences in the habitat within the same geographical location that prevent populations from coming into contact.

This goes back to our example where populations may have different food sources which means it’s less likely they’ll bump into other individuals from the other population and mate.



Temporal isolation:

Temporal means time, and temporal isolation is due to differences in breeding behaviour, such as breeding times or breeding seasons.

If different populations breed at different times it’s unlikely they’ll ever interbreed.

Lizard Species 1

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Behavioural isolation:

Behavioural isolation is a reproductive barrier due to differences in mating behaviours.

Basically, individuals will find the other population too unattractive and so will keep within their own population. This prevents interbreeding from occurring.

While you don’t need to memorise every single type of pre-zygotic RIM you need to at least be able to recognise whether something is a pre- or post-zygotic RIM and why. Otherwise, it’s always good to have a few examples of pre-zygotic RIMs up your sleeve.

STOP AND CHECK:


How pre-zygotic RIMs allow speciation to occur. What kind of pre-zygotic RIMs will be present in sympatric speciation. What kind of pre-zygotic RIMs are due to physical differences between individuals.

What other kind of pre-zygotic RIMs will be present in allopatric speciation (other than ones you’ve already talked about).




Post-zygotic RIMs

Post-zygotic means after the zygote is formed

Therefore, post-zygotic RIMs act after fertilisation has occurred.

As with all RIMs they prevent gene flow.

You would think that it would be difficult to stop different populations interbreeding after the actual breeding has already occurred

But, remember, by definition, different species cannot interbreed to produce a fertile offspring.

So, one way to prevent interbreeding is to either kill off the offspring somehow or make it infertile, both of which stop them from reproducing and passing on their hybrid alleles.

There are 3 main post-zygotic RIMs:

Hybrid inviability:

The hybrid produced from the mating between different species is inviable and dies early in development.

You can’t reproduce and pass on your DNA if you’re dead.

Hybrid sterility:

The hybrid produced from the mating between different species develops normally but is infertile and unable to breed.

So, it can’t pass on its alleles to future offspring.

Hybrid breakdown:

The hybrid produced from the mating between different species may develop normally and breed as well, however, future generations of hybrids are then infertile or inviable.



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Horse Donkey

=

Mule

Can’t Reproduce

STOP AND CHECK:


The difference between pre- and post-zygotic reproductive isolating mechanisms. The 3 main types of post-zygotic RIMs.


Quick Questions

There are four species of crested penguin found in different New Zealand islands, with these islands varying in climate.

These are the features of the different penguins:

Species Size Distinguishing Features Distribution in New Zealand

Snares40 cm, 3

kg

Black, white wite belly. Bright Yellow crest above the eyes droops down back of head. Eyes are slightly red.

The Snares

Fiordland40 cm, 4

kg

Black, with white belly. Bright Yellow crest above the eyes droops down back of head.

Fiordland, and Stewart is.

Erect-crested50 cm, 4

kg

Black, with white belly. Yellow crests above the eyes stick straight up.

Antipodes Is., Bounty Is. and Auckland Is.



Rockhopper40 cm, 3

kg

Black, with white belly. Yellow crests above the eyes stick out at an angle, and almost straight down. Red eyes.

Antipodes Is., Bounty Is., Campbell Is. and

Auckland Is.

How might have speciation of these four crested penguin species occurred? What is the role of reproductive isolating mechanisms? What RIMs may have been present here?

How can pre- and post-zygotic RIMs work together? What are they, and what’s the difference between pre- and post-zygotic ones?

A note on speciation

Remember how mutations randomly occur at approximately the same rate… using DNA evidence from different species you can see which mutations species do and don’t have to estimate how long ago different speciation events occurred; allowing for a phylogenetic trees to be drawn.

PATTERNS OF EVOLUTIONWhen we talk about the patterns of evolution we want to consider the types of evolution and the rates of evolution. So, in this section:

We’ll have a look at 4 types of evolution: divergent and convergent evolution, coevolution and adaptive radiation. We’ll look at why each one occurs and the consequences of each one. We’ll consider evolution that occurs pretty slowly and evolution that occurs pretty fast. Again, we’ll consider why each one occurs and the consequences of each.

Divergent Evolution

The first type of evolution we’ll talk about is divergent evolution, and it is most probably what most people imagine when they think of species evolving.

Divergence, or divergent evolution, occurs when two or more species are formed from a common ancestor.

A good example of divergent evolution is with Darwin’s finches. On Galapagos islands a single species of finch that arrived one day eventually gave rise to 15 different species, all with that same common ancestor.



The evolution of wolves and domestic dogs from a common ancestor is another clear example of divergence.

multiple descendant speciesʼ

ancestral species

extinction

divergencetime

Now that we’ve established what divergence is, we need to think how divergence occurs

An animal from one species doesn’t just decide she might give birth to two new species.

You can generally think of divergent evolution as being the result of allopatric speciation

Remember, allopatric speciation occurs when two populations become geographically isolated and are exposed to different environments and therefore different selection pressures.

Over time the two populations will be so different they are considered two distinct species, arising from the same ancestral species.

Great, we’ve established what divergence is and how it might occur.

But, how do we know if divergence has occurred?

Well, if you see homologous structures you should have a light bulb moment and think “wow, divergent evolution has occurred”.

Homologous structures are basically things that look the same in different animals but don’t work the same. To make it sound a bit more sciency, homologous structures are features which are similar in structure and origin but have different functions.

Let’s break this down:

Similar looking structures tell us that they likely evolved from a common ancestor. Structures which have different functions tell us that they likely evolved in different environments. Common ancestor and different environments should make you think of divergent evolution!



So, what are some homologous structures?

The classic example that biologists froth over is the pentadactyl limb.

Human Cat Bat Whale Horse

In bats it helps them fly, in cats it helps them run and jump, in dolphins it helps for swimming, in horses it helps for running – it’s present in lots of different animals including humans for doing whatever arms and hands do.

This tells us that many vertebrates evolved from a common ancestor.

STOP AND CHECK:


What divergence, or divergent evolution, is. What kind of speciation leads to divergent evolution. The definition of homologous structures. How homologous structures are related to divergent evolution.




Convergent Evolution

Convergent evolution is basically the opposite of divergent evolution.

Rather than two different species' being formed from a single common ancestor and forming different traits, convergent evolution involves two totally different species (different lineages) forming similar traits.

So, we have two or more unrelated species evolving to resemble one another.

This time allopatric speciation isn’t gonna cut it

For species to have similar phenotypes you would expect them to live in the same environment, eat similar foods and have similar lifestyles. And this is exactly how convergent evolution occurs: different species become subjected to the same selection pressures, selecting for the same phenotypes.

Now, they might not look exactly the same, but they might have certain features that are phenotypically similar.

These features are called analogous structures

Analogous structures are the complete opposite to homologous structures: these are features with different evolutionary origins but have similar functions (and may also look kind of similar).

Let’s break it down a bit:

The different origin indicates that these are unrelated species with their common ancestor being very different – remember, all species are even a little related as it is thought that all life originated from one single common ancestor. But with convergent evolution the species are very, very distantly related, like the relationship between humans and jellyfish. The similarities in function tells us that these unrelated species have evolved and live in similar environments, being exposed to similar selection pressures.

Parentspecies

Parentspecies



A good example of an analogous structure is the wings of birds, bats and insects. The wings carry out the same function but often look different between birds, bats and insects. This is because birds, bats and insects are only very distantly related.

Insects Bird Bat

STOP AND CHECK:


The difference between divergent and convergent evolution. How analogous structures are related to convergent evolution.


Co-Evolution

The last type of evolution is co-evolution. With “co-“ you should think “cooperation”, “collaboration”, “coordination”, and so on, because this is all about one species helping another; helping each other evolve, that is.

Co-evolution is a special type of evolution where the evolutionary changes in one species acts as a selection pressure, resulting in an evolutionary change in another species.

The resulting evolutionary change in the other species then acts as a selection pressure on the first species, and the vicious cycle continues. Co-evolution occurs between two species that have an interspecific relationship

There are many possible different interspecific relationships, such as mutualism, parasitism, predation, competition, and so on.

With a predator and its prey, features that help the predator catch its prey will be selected for which then select for features that help the prey run away.



It’s excitingly referred to as an ‘evolutionary arms race’ since survival and reproduction is the ultimate end goal.

STOP AND CHECK:


How co-evolution occurs. The types of relationships that may involve co-evolution.

Try explain it in your own words.

Gradualism and Punctuated Equilibrium

So far, we’ve talked about the types of evolution that may occur, but an important aspect of patterns of evolution is the rate at which evolution occurs.

When the theory of evolution by natural selection was first released, it was thought that all speciation was slow and occurred at a pretty uniform rate.

This idea of gradualism says that the accumulation of continuous small changes over long periods of time resulting in gradual transitions from one form to another.

But there’s a slight issue…

…for some groups of species there was no gradual evolution according to the fossil records.

The opposite of this is punctuated equilibrium

Instead of slow but constant gradual changes, punctuated equilibrium has the style of species remaining stable, with barely any changes occurring, for most of their time of existence (we can call this ‘stasis’).

These long periods of little change are then flipped over with significant evolutionary changes occurring with rapid events – like rapid geological events or climate changes – leading to speciation.

We often liken punctuated equilibrium to “speciation where there have been long periods of little or no change followed by a sudden burst of rapid speciation”.

So, while gradualism likes the idea of species gradually transforming into another, punctuated equilibrium is all for a species splitting into two distinct species.



Which one actually occurs?

Most likely both gradualism and punctuated equilibrium have happened for different groups. Both theories have evidence of occurring in different situations.

Punctuated equilibrium Gradualism

TIM

E

STOP AND CHECK:


What it is meant by the “rate of evolution”. The difference between gradualism and punctuated equilibrium in terms of their rate of evolution.

Which form of speciation gradualism and punctuated equilibrium each produce.


Adaptive Radiation

We end this topic with adaptive radiation, which leads nicely on from divergent evolution. By now you should have it drilled into your brain that divergent evolution involves the splitting of a common ancestor into two different species.

Adaptive radiation is a form of divergent evolution

Specifically, it involves the rapid evolution of a number of species from a single common ancestor.



Those Darwin finches we mention before would fit in with adaptive radiation, because not soon after one species of finch arrived on the islands they produced 15 totally different, distinct species of finch.

Seeds

Buds / FruitLeaves

Insects Grubs

Tool Using Finch

So, what kinds of things do you need for adaptive radiation to occur?

Adaptive radiation occurs when different groups are able to occupy a wide variety of different ecological niches that have only recently become available.

If you think adaptive radiation might be occurring in a neighbourhood near you, look out for a recent ancestry between the species involved and a solid correlation between the phenotype and environment – there needs to be a significant association between the environment and the evolution of traits to provide an advantage in the ecological niche.

STOP AND CHECK:


What makes adaptive radiation different from regular ol’ divergent evolution. Why adaptive radiation involves rapid speciation.




? Quick Questions

Have a look at this phylogenetic tree for Dracophyllum (a type of shrub):

Explain the features of this phylogenetic tree that indicates evolution by punctuated equilibrium.

Dracophyllum milliganii shows evolution by gradualism. What is the difference between gradualism and punctuated equilibrium?

Penguins and seals both have flippers and streamlined bodies for swimming.

What is the pattern of evolution occurring here? How does this type of evolution occur? And how do we know if it has occurred?

Colonies of ants will occupy the bullhorn acacia plant and defends the tree against harmful insects, browsing mammals or hanging vines. The ants also cut and clear vegetation from around the tree. In return, the host supplies the ants with proteins and lipid (called Beltian bodies) from the leaflet tips, as well as carbohydrate-rich nectar from the leaf stalk. The Beltian bodies don’t seem to have a function for the plant.

Discuss the evolutionary relationship between the bullhorn acacia and these ants. First describe the pattern of evolution and explain why this relationship may have developed. What do you think would happen if the ants become reduced in numbers over time?

Compare convergent and divergent evolution.

What are these two types of evolution? Why does each one occur? What are the consequences of each type of evolution? Hint: think homologous and analogous structures.

Australian ancestor24 million years ago Dracophyllum milligan

17 species in New Zealand

9 species in New Caledonia

3 species in Australia

4 species in Australia



KEY TERMSAdaptive Radiation:

A form of divergent evolution characterised by the rapid evolution of a number of different species from a single common ancestor.

Allele frequency: The proportion of a particular allele in the gene pool compared to all other alleles for the same gene/locus.

Allopatric Speciation: Speciation that occurs when the populations are in different geographic locations.

Allopolyploidy: Polyploidy resulting from the hybridisation of genomes from different species.

Analogous Structures: Features which have different evolutionary origins but have similar functions, and may have similar appearances.

Aneuploidy: The state of having an abnormal number of chromosomes in a cell.

Autopolyploidy: Polyploidy involving genome multiplication arising within a single species.

Bottleneck Effect: The change in allele frequencies due to a sudden reduction in population size.

Co-evolution: The type of evolution where the evolutionary changes in one species act as a selection pressure on another, and vice versa.

Convergent Evolution: The process by which unrelated species evolve to resemble one another.

Directional Selection: A mode of selection characterised by selection for one end of the phenotypic range, shifting the whole disruption in one direction.

Disruptive Selection: A mode of selection characterised by selection for the extreme phenotypes and selection against the normal phenotype.



Divergent Evolution: The process by which two or more species are formed from a common ancestor.

Evolution: The change in gene pool of a population over time.

Founder Effect: The decrease in genetic diversity when a small group breaks away from the main population.

Gene Pool: The total set of alleles that are present within a population.

Genetic Drift: The change in allele frequencies due to random sampling and chance.

Gradualism: The process by which speciation occurs at a slow yet constant rate, with accumulation of continuous small changes over long periods of time resulting in gradual transitions from one species to another.

Homologous Structures: Features which are similar in structure and origin but have different functions.

Migration: The movement of individuals from one population to another.

Polyploidy: The state of having more than two sets of chromosomes.

Post-zygotic RIMs: Reproductive isolating mechanisms which prevent gene flow by acting after the egg has been fertilised.

Pre-zygotic RIMs: Reproductive isolating mechanisms which prevent gene flow by acting before the egg is fertilised.

Punctuated Equilibrium: Speciation characterised by long periods of little or no changed followed by a sudden burst of rapid speciation.



Reproductive Isolating Mechanisms: Any factors which prevent individuals from different populations from breeding.

Speciation: The evolutionary process where populations evolve and become distinct species.

Species: A group of related organisms which can breed and produce fertile offspring.

Stabilising Selection: A mode of selection characterised by selection for the average phenotype and selection against the extreme phenotypes.

Sympatric Speciation: Speciation that occurs when the populations are in the same geographic locations.



NOTES

SPECIATION - StudyTime NZfew familiar faces like natural selection, genetic drift, Bottleneck...

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